In general remember red, white, and blue can take heat... Diamond,
Sapphire, and Ruby...That's it.
Better to just remember diamond and corundum. Lots of other white,
red, and blue stones that won’t take much heat. But sometimes you get
surprised. In many cases, it’s not the temperatures themselves that
damage the stones, but the speed of heating or cooling that causes
fractures. That makes sense when you remember that for many stones, a
certain degree of geologic heating was involved in their formation. I
have, for example, managed to now and then heat up a garnet hotter
than should have been possible, without fracturing it. Other times,
they shatter. Same thing, occasionally, with cubic zirconia, and a
few other surprises now and then.
Nothing else can take heat. Especially in silver.
Pretty much true. But Jane, all is not lost when you need to work on
jewelry with a heat sensative stone. While there are various products
sold (“heat sheild”) to protect heat sensative things while
soldering, they all work by being water filled gels or pastes
intended to keep the item cool. Some work OK, others only marginally.
Most suffer from the flaw of having too much structure, so heating
the metal can dry out the interface between metal and protectant,
and then the heat sinking effect, and it’s protection, stops. The
simple answer is to do without the costly products and go back to the
source. Water. If, for example, you have to size a silver ring set
with a nice heat sensative stone like turqoise, or your blue topaz,
you can simply keep the stone under water while you work. In
practice, fill an old tuna fish can or similar container with water,
and position your third hand to hold the ring halfway under water,
so the stone is submerged and the ring shank with it’s cut and fitted
seam, above the water. Now you can solder the seam. It will take a
much larger and hotter flame than you anticipate, because the silver
is sucking the heat from the joint, and the water is sucking the heat
from the silver. In short order, about the time the water in the can
is starting to boil happily, the solder will flow. But the stone
never got hotter than that boiling water, and it can take those
temperatures. Same for almost any other stone you might use. You
started with cold water, ended with boiling water and a stone at
boiling water temps. Let the stone and ring cool in air, not
quenched, since some stones (peridot, for example) might not like the
heat shock from quenching even from just 200F temps.
Alternatives to just the plain water dip are a similar container
filled with very wet sand or soldering grain. still just a can of
water in the end, but with the filler of sand or grain that can help
support the work instead of a third hand. In some cases, the can and
it’s position can be a bother. So then wrap the area needing
protection in tissue paper, hold it on with a bit of wire or thread
or string, or a locking tweezer, and dunk it in water. Be sure the
paper remains soaking wet while you work elsewhere on the ring, and
you’ll have protected the stone from undue heating. Because this
methods holds less water, it’s a bit more limited as to how long and
well it will work, but often, it’s enough.
Although, come to think of it, the synthetic spinels...ie birth
stones...can usually take heat to a degree all except the green
ones which are doublets. Must air cool these though.
You should air cool any stone you’ve heated, including diamonds and
rubies and sapphires. quenching any of them could shatter them. Also,
if heating sapphire or ruby, be sure NOT to allow any flux or boric
acid on the stones, and don’t bathe the stones in the reducing
portions of the flame (best to keep the flame off the stones in any
case). The reason is these stones are aluminum oxide, and fluxes work
to dissolve oxides, so flux or boric acid will damage the surfaces
of the stones, while ordinary oxygen in air will not. Diamond is just
the opposite, of course. Exposed to air, it burns, leaving a frosted
white surface that needs to be repolished by a diamond cutter, so
diamonds must be very clean before any heating, and then protected
with flux or a coating of boric acid to keep oxygen off the diamond.
I have heard star sapphire and star ruby are heat sensitive too,
so I don't suggest risking heating them.
Both of them are easy to damage, since they often have a degree of
internal strain caused by the same inclusions giving the asterism.
While heating is sometimes possible, it’s not a good idea, since
often they’ll crack, even with care. So too, can non-star rubies and
sapphires if heavily included in other ways. In fact, heavily
included diamonds are not always safe either, for the same reason.